As mentioned in the comments, cross-posting between StackExchange sites is explicitly 'forbidden' by network-wide guidelines. The logic to the cross-network posting rule is that each site has a scope and that a 'good' question should be clearly on-topic for one of the sites and thus most likely to get answered in one place. Posting in multiple places is then viewed as likely to simply cause duplication of effort and to potentially fragment useful information.
The StackExchange network policy in part reflects the 'outlook' of the senior staff/owners, who have a particular vision for how it can contribute to knowledge on the internet. The question is whether their logic (and thus guideline) applies to cross-posting in 'other places', for example Usenet, forums, mailing lists, etc., and if so what is the correct response.
Usually, questions are cross-posted because the asker wants to maximise the likelihood of obtaining a (useful) reply. Many 'regulars' here also read/post to 'other places', and so know that cross-posting is not necessarily likely to reach a significantly-larger audience of 'experts' than a single post is. On the other hand, I think many less experienced TeX users do not know that. Thus cross-posting is not a deliberately obstructive action: it reflects the needs/desires of the askers. Thus I think a response here should reflect that: be helpful to the questioner while recognising that duplicated effort does no-one any favours.
I'd therefore suggest that the approach already adopted on The LaTeX Community is not a bad one: politely ask that cross-posts are noted, say as a comment on a question. That makes it more likely that the question will eventually be answered at both ends: someone answering in one place can also answer in the other. Worth noting is that the network approach is that answers here should be 'self-contained', and thus an answer simply pointing out that a cross-post is answered will need to be expanded.